1. Field of the Invention
The present invention relates to electro-optical display devices, and in particular to such devices utilizing a fluoroescently activated plate to collect ambient light and at least one light valve to control transmission of the collected light to a viewer.
2. Description of the Prior Art
Passive optical display devices which are incapable of generating light but which collect and modulate and retransmit the light radiation of the environment in which the display is situated are known in the art. Such devices must be supplied with an external illumination source if it is necessary that the devices provide a display at a visually detectable level under unfavorable light intensity conditions. The use of such external light sources significantly detracts from one of the greatest advantages of such devices, namely the ability to operate at extremely low power input levels. It has thus been a long-standing problem in the art to provide a display device which has a visually detectable output even under unfavorable light input intensity conditions, without significant increase in power consumption.
One solution to this problem has been the development of the so-called "fluorescently activated display" or FLAD. Such devices utilize a fluorescent plate which is located behind a light valve, with the light valve controllable for switching between light transmissive and light blocking states. By appropriate arrangement of switching regions in a light valve which has a surface area coextensive with that of the fluorescent plate, a number of designated patterns can be transmitted to a viewer, with a common application being the representation of digits or numerals in the conventional seven segment configuration.
The fluorescent plate utilized in such devices contains a suitable fluorescent substance therein and is mirrored on its four edges and has indentations or grooves on one surface thereof which is flush with one of the switchable light valve regions. The fluorescent plate collects a large portion of the ambient light which impinges upon the plate and by means of fluorescent scattering and the subsequent reflections from the internally mirrored edges, guides the collected visual radiation and couples it with increased intensity through the indentations. The pattern of the indentations can thus be utilized to transmit information by appropriate transmission and blocking of portions of the pattern by the controlled switching of the light valves.
The light intensity amplification factor of conventional fluorescent plates is given in a first approximation by the ratio of the light collecting plate surface to the light exiting plate surfaces. Further details of the construction and operation of such display devices is found in U.S. Pat. No. 4,142,781 issued to Baur and Greubel.
A problem with the conventional utilization of fluorescently activated devices as described above is that because the fluorescent plate is partially covered by the light valve the plate exhibits only a relatively small collector surface and the efficiency thereof is lessened. Further, some light valves utilized in combination with fluorescent plates operate by polarizing the light transmitted therethrough, thus further decreasing the intensity of the light exiting from the light valves. In general, conventional electro-optical display devices utilizing a fluorescently activated structure have an efficiency of approximately 30%.